Brush wear indicators for dynamoelectric machines, primarily motors with commutators, have previously been suggested. U.S. Pat. No. 4,024,525 shows a construction wherein an axial groove is formed in the edge of the brush and a fixed insulated contact is disposed in this groove which normally touches nothing and an indicator lamp is off. The lamp is illuminated when the brush wears, so that the insulated contact touches the outer end of the groove.
U.S. Pat. No. 2,813,208 discloses a cartridge-type brush holder wherein a brush has a rear central axial aperture and a spring-pressed insulated plunger is urged toward the commutator. When the brush wears, the insulated plunger breaks through the working face of the brush to engage the commutator and push the brush rearwardly away from the commutator to stop the motor.
U.S. Pat. No. 3,523,288 discloses a structure wherein a continuous electrical circuit is normally made through a lamp to energize it while the brush length is satisfactory. This circuit is made through a spring contact arm on the side of the brush holder and through a pin to the side of the brush. When the brush wears sufficiently, the pin drops into an aperture in the side of the brush and the lamp is then de-energized.
U.S. Pat. No. 4,172,988 discloses an extra contact arm electrically and physically connected to the rear axial end of the brush. Upon sufficient brush wear, this moving contact arm engages a fixed contact to energize an indicator.
U.S. Pat. No. 2,691,114 illustrates an extra cantilever contact arm which is insulated from but moves with a brush spring arm which urges the brush toward the commutator. Upon sufficient brush wear, the extra cantilever contact arm engages the brush holder to illuminate an indicator lamp.
U.S. Pat. No. 4,121,207 discloses a roller actuator on a fixed miniswitch and the roller will roll over the outer axial end of the brush upon sufficient wear thereof to close the switch and illuminate an indicator lamp.
The first four of the above-mentioned patents have the deficiency that if the indicator lamp is ignored, the brush wear indicator will hold the brush away from the commutator and thus stop operation of the motor. This may be very disadvantageous where interruption of the motor rotation would be highly undesirable. Even the fifth-mentioned patent has this disadvantage because the force with which the extra cantilever arm engages the brush holder is effectively subtracted from the force with which the spring arm urges the brush toward the commutator, and upon sufficient further wear the brush will no longer be urged toward the commutator. The difficulty with the last-mentioned patent is the relatively high cost of the separate miniswitch plus the difficulties of mounting such a switch on parts of the motor which may be at a high voltage, for example, 700 volts.